The amount of current in a CFL rated 23W-120V is about
A) 0.2A.
B) 5.2A.
C) more than 5.2A.
Answer: A
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Draw and label a simple diagram of the following: a. a refracting telescope b. a reflecting telescope
What will be an ideal response?
A homeowner wants to replace an electric hot-water heater. There are two models in the store. The inexpensive model costs $280 and has no insulation between the inner and outer walls. Due to natural convection, the space between the inner and outer walls has an effective conductivity of 3 times that of air. The more expensive model costs $310 and has fiberglass insulation in the gap between the walls. Both models are 3.0 m tall and have a cylindrical shape with an inner wall diameter of 0.60 m and a 5 cm gap. The surrounding air is at 25°C, and the convective heat transfer coefficient on the outside is 15 W/(m2 K). The hot water inside the tank results in an inside wall temperature of 60°C. If energy costs 6 cents per kilowatt-hour, estimate how long it will take to pay back the extra
investment in the more expensive hot-water heater. State your assumptions.
GIVEN
FIND
- The time it will take to pay back the extra investment in the more expensive hot-water heater
ASSUMPTIONS
- Since the diameter is large compared to the wall thickness, one-dimensional heat transfer is
assumed
- To simplify the analysis, we will assume there is no water drawn from the heater, therefore the
inside wall is always at 60°C
- Steady state conditions prevail
SKETCH
PROPERTIES AND CONSTANTS
fiberglass (ki) = 0.035 W/(m K) at 20°C
dry air (ka) = 0.0279 W/(m K) at 60°C
If the zeroth law of thermodynamics were not valid, which of the following could not be considered a property of an object?
A. Pressure B. Center of mass energy C. Internal energy D. Momentum E. Temperature
You observe the peak brightnesses of two white dwarf supernovae. Supernova A is only 1/4 as bright as Supernova B. What can you say about their relative distances?
A) Supernova A is twice as far away as Supernova B. B) Supernova A is 4 times as far away as Supernova B. C) Supernova B is 4 times as far away as Supernova A. D) Supernova B is twice as far away as Supernova A. E) We can't say anything about their relative distances because we do not have enough information.